1. Field of the Invention
This invention relates to gas turbine engines and, more particularly, to a simplified method for reducing nitrous oxide emissions through the technique of water injection.
2. Description of the Prior Art
In this era of environmental awareness it is anticipated that regulations covering air pollution will become increasingly restrictive and that compliance with industrial emission standards will become more difficult to attain. These environmental considerations will have an impact upon the development of industrial gas turbine engine power plants and may require the reduction of exhaust emission levels consistent with available technology at realistic costs. However, the trend in gas turbine engine development is toward higher temperature engines which, though they are inherently more efficient, also tend to produce higher emission levels of nitrous oxide (NO.sub.x).
It is generally accepted that NO.sub.x formation increases exponentially with flame temperature. It has also been generally acknowledged that NO.sub.x formation can be reduced by introducing water in the form of liquid or steam into the combustion process to reduce the temperature to which the air is heated by the combustion of fuel in the primary zone of combustion. Because of the exponential increase of NO.sub.x formation with flame temperature, relatively large reductions in NO.sub.x can be achieved with relatively low water flow rates. Furthermore, the specific method of water injection in gas turbine engines does not appear to be particularly important. Water has been injected separately into the combustor through distinct water nozzles as a liquid and as steam. It has also been injected into the combustor in the upstream, downstream and "side stream" directions through separate water passages in the fuel injector. It has even been introduced through dual-flow nozzles wherein the water and fuel were injected coaxially into the combustor. However, although these methods of injecting water have been successful in controlling NO.sub.x, they have, on occasion, produced some problems with hardware life due to local temperature gradients in the region where the water is being injected. In fact, instrumented sector tests have demonstrated that in using the upstream method of injecting water through the nozzle, combustor metal temperature variations increase from a normal 260.degree. C. temperature variation with no water injection to a 427.degree. C. variation with the amounts of water injection necessary to achieve significant NO.sub.x reductions. While these temperature variations are the measured results of one particular series of engine sector tests, they are representative of the trend in temperature variations to be found in other gas turbine engine combustors.
More recently, a concept for emulsifying the fuel and water together and injecting the mixture through the normal (or enlarged) fuel nozzles has been used successfully. This has considerable advantages over the systems relying on separate injection of fuel and water since complexity is minimized, separate nozzles may be eliminated, and costs reduced accordingly.
There is an old axiom that fuel and water won't mix. However, they will--but only temporarily. They then separate at a rate that appears to be a function of the specific gravity of the fuel. As the specific gravity approaches unity (where fuel has the same density as water), the separation rate becomes much slower. To achieve satisfactory fuel-water emulsion, current practice has been to process the two separate liquids through a homogenizer where each is pressurized to a very high level and then sprayed through extremely small orifices into impingement against a hard impact block in a common mixing chamber. The impact breaks each fluid into extremely fine particles which become intimately mixed, or emulsified, into one homogeneous fluid. The subsequent separation rate is apparently slowed by the intimacy or fineness of the emulsion. This homogenizing equipment is, of course, very bulky and costly.
Since water suppression of NO.sub.x is simply a function of water concentration, the emulsion concept is only one means employed to assure that each fuel nozzle is supplied with the same quantity of fuel and water as are all the others. Since all nozzles are supplied by a common pressure source (usually a fuel manifold), then all will flow the same rate of fluid, be it fuel, water or a fuel-water emulsion. If separation occurs prior to combustion, then some nozzles will flow more fuel (or water) than others and unacceptable temperature distributions will result inside the engine. In fact, it has been found that fuel variations between nozzles in excess of 10 percent are generally undesirable. In short, the fuel and water need be mixed or emulsified only to the extent required to assure uniform distributions throughout the manifolded fuel nozzles. Since state-of-the-art fuel-water emulsifiers or homogenizers are inherently complex, heavy and costly, it would be advantageous to develop a simple emulsifier which merely meets requirement of uniform fuel distribution among the manifolded nozzles.